Replying to my own post: further examination shows one of the high voltage terminal posts on the MSD coils has cracked, likely as a result of arcing when wet. No way that's getting patched up. Replacements shall arrive tomorrow, then full reassembly.
Here's a helpful tip for those who have Kingsborne ignition leads. The right-angle boot on the coil end is too large in diameter to seal tightly, and/or too short to reach all the way down the post where the post is fatter. On
MSD 8224 coils, there's a gap of about 1mm. When I reinstall, there'll be a judicious amount of silicone dielectric grease on both the posts and the boots, to hopefully fill the gap and prevent the problem.